There are three generally accepted methods for reducing switching losses associated with a solid state switching device: (1) use of resonant mode switching; (2) use of an active snubber circuit; and (3) use of an inductor in series with the switching device and a snubber capacitor in parallel with the switching device.
Resonant mode switching turns on the switching device for a fixed amount of time. Output power is controlled by varying the operating frequency of the switching device. However, varying the operating frequency is more likely to excite some parasitic power circuit oscillations. Also, varying the frequency makes filtering out ripple voltage or ripple current more difficult.
Use of an active snubber circuit which includes additional power components, heat sinks and control circuitry, increases cost and complexity of the switching circuit.
FIG. 1 illustrates a typical step down switch mode (buck chopper) power supply 101, utilizing a switching circuit with switching device SW and series inductor L10. Switching device SW can be any suitable solid state switching device, such as a bipolar transistor, insulated gate bipolar transistor (IGBT) or field-effect transistor. Terminals 1 and 2 are connected to the output of a suitable dc power source (not illustrated in the figure). Capacitor C10 serves as an ac ripple filter for the output voltage of the dc source. Freewheeling diode D10 which is selected from a class of diodes commonly known as fast recovery diodes, provides current flow to series inductor L20 when device SW is turned off. Diode DS, resistor RS and capacitor CS form a snubber circuit that carries current when device SW initially turns off. Load RL is connected to terminals 4 and 5. Use of series inductor L10 reduces stress on switching device SW by preventing simultaneous high voltage and high current during the critical turn-on time of the switching device. Additionally the series inductor reduces stress on freewheeling diode D10 by controlling the rate of change of the reverse current when the diode transitions from forward bias to reverse bias and stops conducting current. However, the series inductor has negative effects when switching device SW turns off. Series inductor L10 causes a high voltage to appear across switching device SW during turn-off. Inductor L10 also reduces the rate of change of current in freewheeling diode D10 after the switching device turns off. This results in diode D10 not carrying the full current immediately after switching device SW turns off. Because inductor L20 forces current to flow, some current path must be provided. Generally, a second diode D20 in series with resistor R20 is required to provide a parallel path for the current while current ramps up in D10 for a relatively long time period of 8 to 10 microseconds (μs). The problems associated with the use of a series inductor can be overcome at the expense of relatively high switching losses and additional circuitry.
Therefore, there is the need for a solid state switching circuit that utilizes an inductive impedance in series with the switching device and freewheeling diode that has low switching circuit losses.